High speed wireless communications over high frequency bands suffer severe path loss, and thus require high gain antennas. Existing methods to enable high antenna gain include use of directional antennas and use of antenna arrays. The latter is often preferred because a beam direction can be adaptively steered in an electronic manner. Antenna array beamforming (BF) provides increased signal quality due to high directional antenna gain. Further, steering the transmitted signal in a dedicated direction extends the communication range.
A beamforming operation can be implemented in an analog domain, after a digital-to-analog converter (D/A or DAC) at a transmit station and before an analog-to-digital converter (A/D or ADC) at a receive station. Beamforming can also be implemented in the digital domain, before the D/A at the transmit station and after the A/D at the receive station.
Digital beamforming is proposed in the 802.11n draft specification (“Draft Amendment to Standard for Information Technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Enhancements for Higher Throughput,” IEEE P802.11n/D1.0, March 2006). Digital beamforming using eigen-decomposition is an optimal approach, providing the highest throughput and reliability.
However, digital beamforming is a costly scheme because it requires the same number of radio frequency (RF) chains as the number of antennas in an antenna array. Analog beamforming, on the other hand, requires one RF chain for an antenna array. For applications such as 60 GHz frequency band wireless networks (e.g., wireless high-definition or WirelessHD), since the bandwidth is high (the spectrum efficiency is not high), reliable transmission of one data stream via an RF chain over an antenna array is satisfactory. Analog beamforming provides a simple solution to reduce the RF chain cost while maintaining the array gain.
In a related iterative analog beamforming scheme, a beam search protocol is based on explicit feedback of a transmit beamforming vector between a first station STA1 (a BF transmitter) and a second station STA2 (a BF receiver). To estimate the optimal transmit beamforming vector, the transmitter uses switch beamforming and switches across all possible beams. This approach is useful in finding a transmit beamforming vector in the initial stage of a beamforming search protocol. However, if a useful initial estimate of the receive beamforming vector is required, then such an approach cannot be applied because switching across all transmit beams cannot be performed for estimating the optimum receive beamforming vector, while maintaining a sufficient link budget.